This invention relates to a process for the self-feeding combustion of a mixture of sewage sludge and polymeric TDI residues in fluidized bed furnaces.
A multi-stage process using TDI residues that result from the production of monomeric tolylene diisocyanates ("TDI") has been described in DE-OS No. 2,920,526 as providing improved disosal of sewage sludges of the kind obtained as surplus clarification sludges in biological clarification plants. Improvements in the settling processes of sewage sludges and improvement in partial sludge dewatering are described for this process. The reference also describes the combustion of sewage sludges mixed with TDI residues in fluidized bed furnaces, using quartz sand as fluidized bed material. According to this reference, combustion of mixtures which have a solids content of up to 32.2% by weight can be achieved only by using considerable quantities of energy sources such as coal, natural gas or fuel oil.
Any increase in the solids content was considered impossible, however, because of concerns that the level of nitrogen oxides in the fuel gas would rise above the permissible limit due to the high nitrogen content in the sewage sludges (about 5 to 8% by weight) and the TDI residues (16.8 to 17.2% by weight). It was therefore not to be expected that one could burn improved mixtures of sewage sludges and a high proportion of nitrogen-containing TDI residues without supplying the usual energy sources and without producing environmental and processing problems.
It was, therefore, all the more surprising to find that aqueous sewage sludges containing TDI residues could be burned trouble-free under environmentally benign conditions by using a completely self-feeding fluidized bed process in which the concentration of solids in the mixture and the concentration of TDI residue are kept within certain limits.